1. Field of the Invention
This invention relates to an apparatus for aligning a mask and a substrate (particularly a semiconductor wafer) relative to each other, and in particular to an alignment apparatus suitable for an apparatus for exposing the pattern of a mask to a photosensitive substrate.
2. Related Background Art
In an apparatus for exposing the pattern of a mask to a photosensitive substrate, it is requisite to highly accurately accomplish the work of optically detecting a pattern (or a mark) preformed on the photosensitive substrate and a pattern (or a mark) on the mask and positioning the mask or the photosensitive substrate so that the two patterns (or marks) are correctly superposed one upon the other, that is, the so-called alignment. In recent years, in the field of the exposure apparatus used to make a semiconductor element on a wafer, the stepper which is made into a system for projecting the pattern of a mask onto a small area on a wafer through a projection lens and in which the wafer is caused to effect stepping for the exposure of the whole surface of the wafer has become the main current. This stepper is such that a circuit pattern formed on a reticle as a mask is projected onto a wafer by a projection lens of high resolving power and high N.A. The alignment in the stepper is accomplished by reversely projecting an alignment mark on the wafer onto the reticle side through the projection lens, observing the alignment mark on the reticle and the spatial image of the alignment mark of the wafer (which is formed on the reticle surface) at a time, and detecting the positional deviation between the two marks. In this case, it is usual to design the illuminating light for observing (detecting) the marks so as to enter the projection lens and the wafer through the reticle. The alignment optical system (including the illuminating system) in such a conventional alignment apparatus is described in detail, for example, in U.S. Pat. No. 4,402,596.
The construction of this conventional apparatus is such as shown, for example, in FIG. 9 of the accompanying drawings wherein a projection lens which is telecentric on the wafer W side (the image side) and non-telecentric on the reticle R side (the object side) is used as a projection lens PL. A pattern PA to be superposedly printed on the wafer W and an alignment mark RM are formed on the reticle R. The wafer W is placed on a two-dimensionally movable stage ST, and an alignment mark WM matching the mark RM is formed on the wafer W. In FIG. 9, the light ray L passing through the center of the pupil ep of the projection lens PL represents the principal light ray of the illuminating system for exposure. The alignment system is constituted by a light source 1 as illuminating means, a beam splitter 2, a second objective lens 3, a first objective lens 4 and a total reflection mirror 5 (and further, an illuminating field stop, not shown). The image of the RM of the reticle R is formed on an imaging surface 6 through the objective lenses 4 and 3, and the image of the mark WM of the wafer W is once formed in the same surface as the mark RM of the reticle R through the projection lens PL, whereafter it is again formed on the imaging surface 6 by the objective lenses 3 and 4. The light-receiving surface of an image pickup device such as a television camera is positioned on the imaging surface 6, and the images of both marks RM and WM are photoelectrically detected at a time. In FIG. 9, the line l.sub.1 passing through the mark WM of the wafer W, the center of the pupil ep and the mark RM of the reticle R represents the principal light ray of this alignment optical system, and the objective lenses 3 and 4 of this alignment optical system are used eccentrically. The image of the light source 1 may be formed on the pupil ep of the projectinn lens PL.
When, as shown in FIG. 9, the illuminating light for alignment is caused to enter from the opposite side of the projection lens PL with respect to the reticle R and illuminate the mark RM, the illuminating light passed through the transparent portion around the mark RM travels along the principal light ray l.sub.1 and illuminates a localized area including the mark WM of the wafer W. Simultaneously therewith, the image of the mark RM is formed on the wafer W. Usually, photoresist is applied to the surface of the wafer W and this surface has reflectivity for the illuminating light for alignment. Therefore, assuming that the localized area including the mark WM is perpendicular to the optic axis AX of the projection lens PL, the principal light ray l.sub.1 is parallel to the optic axis AX on the wafer W side and thus, the image of the mark RM formed on the surface of the wafer W is reflected by the wafer W and is reversely projected by the projection lens PL so as to again overlap the mark RM. Of course, the image of the mark WM on the wafer is also formed on the transparent portion aoound the mark RM by the projection lens PL.
Now, the object side (the reticle side) of the projection lens PL is non-telecentric, and by the objective lenses 3 and 4 being used eccentrically, the regularly reflected light on the mark RM travels in the direction of arrow CA and does not return toward the mirror 5 when the llluminating light from the light source 1 is bent by the mirror 5 and illuminates the mark RM of the reticle R. Such a construction can be realized by obliquely disposing the mirror 5 at an angle of 45.degree. with respect to the surface of the reticle (when the objective lenses 3 and 4 are disposed horizontally) with the fore end portion of the mirror 5 being made substantially coincident with the optic axes of the objective lenses 3 and 4 so that the illuminating light (or the alignment light from the mark may pass through the half area of the objective lenses 3 and 4. Thus, the mark RM of the reticle R is illuminated by the reflected light of the illuminating light for alignment reflected by the wafer W and, if the pattern of the mark RM is of a light-intercepting property, the mark RM is imaged as a dark portion on the image pickup device.
However, paying attention to the image of the mark RM formed on the imaging surface 6, this image is in some cases formed by two images of different properties (but of the same shape) being superposed one upon the other. Assuming here that the off-axis aberration of the projection lens PL is ideally zero and that the localized area including the mark WM of the wafer W is an ideal reflecting plane perfectly perpendicular to the principal light ray l.sub.1, the image of the mark RM formed on the wafer W is reflected by the wafer W and is re-imaged at a position whereat it accurately overlap the mark RM. Therefore, an image equal to the image formed when the mark RM of the reticle R is simply illuminated is sharply formed on the imaging surface 6 with good contrast, and thus, during the alignmnnt thereof with the mark WM on the wafer, the pattern edge of the mark RM can be detected precisely. Actually, however, the ideal conditions as supposed above do not exist, and if the aberration of the projection lens PL and the inclination of the surface of the wafer W with respect to the optic axis of the projection lens PL deviate greatly from the ideal conditions, the reversely projected image of the image of the mark WM of the wafer W onto the reticle R will become bad in contrast and will not accurately overlap the mark RM. Therefore, the pattern edge of the mark will become unclear, and this has led to the problem that the mark detection accuracy during alignment is reduced. The dual image as described above has readily appeared particularly when the reticle R and the wafer W are not accurately conjugate (in-focus) with respect to the projection lens PL or when the telecentricity of the alignment system including the projection lens goes slightly wrong. There has also been the problem that the images of the mark RM and the mark WM overlap each other and the two images cannot be discriminated from each other with a result that much time is required for alignment.